This invention relates to an ignition timing system for a spark ignited combustion engine for a motor vehicle and particularly to such a system utilizing a closed loop knock feedback control. Such systems generally provide knock correcting ignition retard in response to sensed knock in order to maintain knock at trace levels.
Most knock controlling ignition timing systems utilize a vibration sensor for detecting the engine vibrations caused by knock events in the combustion chambers and generating an output knock signal when such vibrations occur. Such sensors provide a cost advantage in that a single sensor is capable of sensing knock from all the combustion chambers in the engine. However, a problem with using an engine mounted vibration sensor for detecting knock is that it will also sense other vibrations of the engine due to movements of engine components, such a valve opening and closing and piston slap, many of which vibrations are unpredictable from engine to engine or from cycle to cycle. Such other vibrations constitute noise in the knock signal.
Improvement of the signal to noise ratio in knock detection by a vibration sensor has been given much attention by engine control engineers; and the prior art shows many systems and techniques for dealing with the problem. The development of technology in this area has been sufficiently successful that many vehicles are produced with knock control systems providing adequate to excellent knock control. However, many of the vibrations produced by noise still cannot be clearly distinguished from those produced by knock events; and the sensing of noise vibrations as knock can lead to unnecessary ignition retard with an accompanying reduction in engine performance, especially with respect to fuel economy. The smaller, higher speed engines which are becoming more popular as fuel economy expectations increase, and especially those with multiple valves per combustion chamber, appear to be especially noisy. Many of these engines severely test the limits of the present knock control technology. In today's environment, any technique which can gain an additional improvement in signal to noise ratio for a knock signal for a particular engine has the potential of providing a fuel economy increase.
One known method of noise reduction for a knock sensor of the vibration sensing type is that of knock windowing. Since knock is expected during a particular portion of the engine cycle for each combustion chamber and is much less likely to be found during the remainder thereof, the ignition timing system is thus made responsive to the knock sensor only during a predetermined crankshaft rotational angle in which knock, if present, is expected. Thus, the proportional contribution of noise to a knock signal derived from the sensor output is reduced. However, in some engines, there is a possibility of reducing the proportional contribution of noise in the knock signal even further.